CC10 — Environmental Chemistry

📖 CC10 · CDS General Science — Chemistry

Environmental chemistry links chemistry to the world around us — explaining why the ozone layer is thinning, why acid rain damages buildings, and how greenhouse gases trap heat. CDS tests these connections directly through factual questions on pollutants, their sources, and effects.

📌 CDS Focus: Greenhouse gases (CO₂, CH₄, N₂O, CFCs) and global warming; ozone layer depletion by CFCs (Montreal Protocol); acid rain causes (SO₂ + NO₂); BOD as a measure of water pollution; particulate matter (PM₂.⁵, PM₁₀); smog = smoke + fog; photochemical smog = NO₂ + hydrocarbons + sunlight.

1. Air Pollution

Fig. 1 — Major Air Pollutants: Source, Effect and Example

2. Greenhouse Effect & Ozone Layer

🌡️ Greenhouse Effect & Global Warming

Greenhouse gases trap infrared radiation from Earth's surface
Main greenhouse gases: CO₂ (most significant), CH₄ (methane), N₂O, H₂O vapour, CFCs
Enhanced greenhouse effect leads to global warming
Consequences: melting glaciers, sea level rise, extreme weather
Kyoto Protocol (1997) and Paris Agreement (2015) — international treaties
CO₂ level (pre-industrial): 280 ppm; now exceeds 420 ppm

🌍 Ozone Layer Depletion

Ozone (O₃) layer in stratosphere (15–30 km altitude) absorbs harmful UV-B radiation
CFCs release Cl atoms which destroy ozone: Cl + O₃ → ClO + O₂
One Cl atom destroys up to 100,000 ozone molecules
Ozone hole: mainly over Antarctica (Antarctic vortex)
Montreal Protocol (1987): International agreement to phase out CFCs
Replacements: HFCs (hydrofluorocarbons)

3. Water & Soil Pollution

Water Pollution Key Terms for CDS:

● BOD (Biochemical Oxygen Demand): Amount of O₂ consumed by microbes to decompose organic matter in water. High BOD = heavily polluted water. Clean water BOD < 5 ppm; sewage BOD > 200 ppm.
● Eutrophication: Excess fertiliser runoff causes algae bloom → depletes O₂ → kills fish (dead zones).
● Hardness of water: Caused by Ca²⁺ and Mg²⁺ salts. Temporary hardness (carbonates) removed by boiling; permanent hardness (sulphates/chlorides) removed by washing soda (Na₂CO₃) or zeolite process.
● Soil pollution: Excess pesticides (DDT, BHC), plastic waste, heavy metals accumulate and reduce soil fertility.

📝 CDS PYQs — Environmental Chemistry

Q1. Acid rain is primarily caused by: CDS PYQ

(a) CO₂ and CO(b) SO₂ and NO₂(c) CFCs and O₃(d) CH₄ and N₂O

✔ Answer: (b) SO₂ and NO₂

Acid rain is caused when SO₂ (from coal burning) and NO₂ (from vehicle exhaust) react with water vapour in the atmosphere to form H₂SO₄ (sulfuric acid) and HNO₃ (nitric acid). Normal rain pH ≈ 5.6 (slightly acidic due to CO₂); acid rain pH < 5. Effects: damages limestone monuments, kills aquatic life, harms forests.

Q2. The Montreal Protocol was designed to control the production of: CDS PYQ

(a) CO₂(b) CFCs(c) SO₂(d) CO

✔ Answer: (b) CFCs

The Montreal Protocol (1987) is an international treaty to phase out the production and use of ozone-depleting substances, primarily CFCs (chlorofluorocarbons). CFCs were widely used as refrigerants and aerosol propellants. They release chlorine atoms in the stratosphere that catalytically destroy ozone molecules. The Protocol is considered the most successful environmental treaty in history.

Q3. High BOD (Biochemical Oxygen Demand) in water indicates: CDS PYQ

(a) Clean water(b) High oxygen content(c) High organic pollution(d) Presence of minerals

✔ Answer: (c) High organic pollution

BOD measures how much oxygen microorganisms need to break down organic matter in water. High BOD = heavily polluted with organic waste (sewage, industrial effluent). The decomposition consumes dissolved O₂, depleting it from the water and making it unable to support fish and aquatic life. Clean rivers have BOD < 5 mg/L; heavily polluted water can exceed 200 mg/L.

🧠 Quick Memory Chart — CC10

🌡 Greenhouse Gases

CO₂: fossil fuels (most significant)
CH₄: cattle, rice paddies
N₂O: agriculture, fertilisers
CFCs: refrigerants (banned)
Montreal (ozone); Kyoto (GHGs)

🌊 Water Pollution

BOD: high = polluted
Eutrophication: algae bloom kills fish
Temporary hardness: boiling
Permanent hardness: Na₂CO₃
DDT: persistent soil pollutant

🚨 Acid Rain

Caused by SO₂ + NO₂
Forms H₂SO₄ + HNO₃ in clouds
pH < 5; damages marble, Taj Mahal
Ozone hole: Antarctica; CFCs
Smog = smoke + fog

📝 Practice Exercise

E1. The main greenhouse gas responsible for global warming is:

(a) CH₄(b) CO₂(c) N₂O(d) CFC

E2. Photochemical smog is formed by the reaction of:

(a) SO₂ + water(b) NO₂ + hydrocarbons + sunlight(c) CO + fog(d) Smoke + rain

E3. Temporary hardness of water is removed by:

(a) Adding Na₂CO₃(b) Boiling(c) Adding alum(d) Filtration

Answers: E1 → (b) CO₂ [contributes ~76% of enhanced greenhouse effect; CH₄ is more potent per molecule but less abundant] | E2 → (b) NO₂ + hydrocarbons + sunlight [produces ozone + PAN (peroxyacetyl nitrate) near ground level] | E3 → (b) Boiling [Ca(HCO₃)₂ → CaCO₃ precipitate + H₂O + CO₂; permanent hardness needs Na₂CO₃]

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